The present invention relates to acid prepared mesoporous silica spheres and methods of synthesizing the same.
Porous silica is commonly use as a matrix material for chromatographic separations. With surface areas in the neighborhood of 300 m2/g, commercially available chromatographic grade silicas possess a relatively high surface area. Mesoporous materials, which typically possess surface areas in excess of 1000 m2/g and even as high as 1600 m2/g, are commonly used as adsorbents, catalysts, and catalytic supports. With such high surface areas, these materials should provide superior separating ability as a chromatographic matrix in liquid chromatography (LC), flash liquid chromatography (FLC), and high performance liquid chromatography (HPLC).
Various techniques exist for synthesizing mesoporous silica. For example, U.S. Pat. No. 4,554,211 to Arika et al., discloses a technique for synthesizing mesoporous silica spheres using an emulsion templating mechanism in basic solution. Other patents describing techniques for synthesizing large pore oxides in basic solution include U.S. Pat. No. 5,068,216 to Johnson et al., U.S. Pat. No. 5,168,828 to Degnan et al., and U.S. Pat. No. 5,308,602 to Calabro et al. Recently, processes have been developed for synthesizing mesoporous silica spheres in acidic solution. In an article by Stucky et al., Oil-water Interface Templating of Mesoporous Macroscale Structures, Science, 1996, 273, 768-771 an emulsion process for synthesizing mesoporous silica spheres was described. A silicon alkoxide (TEOS) was dissolved in an organic solvent, typically mesitylene. This mixture was added, slowly over a period of 30 minutes, to an aqueous acidic solution containing a cationic ammonium surfactant (CTAB). Stucky found that by varying the stir rate during the course of the reaction, the particle morphology could be changed. At slower stirring rates, the reaction mixture produced fibers, and as the stirring rate was increased, the amount of fibers decreased with the increasing amounts of spheres. It was shown that the size of the spherical particles decreases with increasing stirring rates. Scanning Electron Microscopy (SEM) indicated the particles were hollow and spherical in nature. It was shown that these hollow spheres were brittle, and could be crushed with a spatula. The brittle nature of the spheres, in combination with the fact that they were not porous throughout their interior, seemed to indicate unfavorable characteristics for their use as a chromatographic matrix.
Qi et al., in the article Micrometer-Sized Mesoporous Silica Spheres Grown Under Static Conditions, Chemistry of Materials, 1998, 10, 1623-1626, describes the formation of mesoporous silica spheres by a process using a cationic-nonionic surfactant mixture in aqueous acidic conditions. A typical synthesis involved stirring an aqueous acidic solution of a cationic ammonium surfactant (CTAB), and a nonionic surfactant (decaethylene glycol monohexadecylether), to which an alkoxysilane was added (TEOS). This material was presumably porous throughout its interior, although this was not specifically addressed in the article. The material seems to possess desirable characteristics, a high surface area (1042 m2/g) and xcx9c5 xcexcm particle size, for use as a chromatographic matrix, but the long synthesis time (16 hours) and the use of a mixture of surfactants rather than one does not seem desirable for use on a commercial scale.
Yet another process for synthesizing mesoporous silica spheres in acidic aqueous solution is described by Ozin et al., in the article Synthesis of Mesoporous Spheres Under Quiescent Aqueous Acidic Conditions, Journal of Materials Chemistry, 1998, 8(3), 743-750. An acidic aqueous solution consisting of an alkoxysilane (TEOS) and a cationic ammonium surfactant (CATCl), was allowed to react under static conditions for a period of 7-10 days at 80xc2x0 C. It was also demonstrated that spherical particles could be synthesized at room temperature with a modified reaction imixture. Particle sizes appeared to range from 1-30 xcexcm. While the spheres Ozin et al. produced are monodisperse, the lower surface area (750 m2/g) and long synthesis time (7-10 days) makes the material and process unattractive for use on a commercial scale. All of the processes described above produce materials which exhibit regular powder X-ray diffraction patterns with one or more relatively narrow diffraction peaks. This indicates that they contain a relatively ordered arrangement of pores. It appears that the materials produced by these processes are similar to SBA-3, a mesoporous material with a hexagonal arrangement of linear pores (xe2x80x9cMesostructure Design with Gemini Surfactants: Supercage Formation in a Three-Dimensional Arrayxe2x80x9d, Huo et al., Science, 1995, 268, 1324). SBA-3 is similar to the more widely known MCM-41, which has an identical arrangement of pores but is synthesized in basic solution (xe2x80x9cOrdered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Templating Mechanism,xe2x80x9d Kresge et al., Nature, 1992, 359, 710). While mesoporous silica having such ordered pores has use in a variety of contexts, the processes for synthesizing such materials tends to take longer, or be more complex, than is commercially desired. The present invention relates to acid-prepared mesoporous silicates and methods of synthesizing same.
One aspect of the present invention is a method of forming mesoporous inorganic oxide particles in which at least 50% of the particles are spherical. The method involves first preparing a reaction mixture capable of forming said mesoporous inorganic oxide particles.
The reaction mixture comprises:
i. a mineral acid selected from the group consisting of HCl, HBr and HI;
ii. an inorganic oxide source consisting of a compound having a formula Si(OR1)(OR2)(OR3)(OR4) where Si is silicon, O is oxygen and R1, R2, R3 and R4 are alkyl chains having 1 to 4 carbon atoms;
iii. a surfactant consisting of one or more members of the group consisting of:
(1) a cationic ammonium having a formula R1R2R3R4N+Xxe2x88x92, where R1, R2 and R3 are alkyl chains consisting of 1 to 6 carbon atoms, R4 is an alkyl chain consisting of 12 to 24 carbon atoms and Xxe2x88x92 represents a counterion to said surfactant selected from the group consisting of Cl-, Br-, I- and OH-;
(2) a cationic diammonium having a formula [R1R2R3N+R4N+R5R6R7]Xxe2x88x92Xxe2x88x92, where R1, R2, R5 and R6 represent alkyl chains consisting of 1 to 6 carbon atoms, R3 represents an alkyl chain of 12 to 24 carbon atoms, R4 represents an alkyl chain of 3 to 16 carbon atoms, R7 represents an alkyl chain of 1 to 24 carbon atoms, and Xxe2x88x92 represents a counterion to the surfactant which may be Cl-, Br-, I- or OH-; and
(3) a tri-block copolymer EOxPOyEOz, where EO is polyethylene oxide, PO is polypropylene oxide and x ranges from 5 to 106, y ranges from 30 to 85 and z ranges from 5 to 106; and
iv. water.
As the next step in the method, the reaction mixture is mixed sufficiently so that mesostructured inorganic oxide particles may be formed in a subsequent heating step. This latter step involves heating the reaction mixture at a temperature and for a time sufficient to form mesostructured inorganic oxide particles, at least 50% of which are spherical. Finally, organic material is removed from the mesostructured inorganic oxide particles so as to form mesoporous inorganic oxide particles.
Another aspect of the present invention is a method of performing a liquid chromatographic separation of a liquid or dissolved solid compound using the mesoporous inorganic oxide synthesized using the process described above. This method involves packing a chromatography column with a slurry of such mesoporous inorganic oxide. The slurry includes an organic solvent selected as a function of the liquid or dissolved solid compound to be separated. Next, the liquid or dissolved solid compound is added to the slurry of mesoporous inorganic oxide. Finally, a mobile phase of the liquid or dissolved compound is retrieved from the chromatography column.
These and other aspects of the present invention are described in more detail below, are illustrated in the accompanying drawings and are set forth in the appended claims.